1. Field of the Invention
The invention relates to a feedback control circuit and a power converting circuit, and particularly to a feedback control circuit and a power converting circuit capable of reducing overshoot.
2. Description of Related Art
FIG. 1 is a schematic view of a conventional light emitting diode (LED) driving circuit. Referring to FIG. 1, the LED driving circuit includes a controller 10, a converting circuit 50, and an LED module 60. The converting circuit 50 is coupled to an input voltage source Vin. The controller 10 generates a control signal Sc to control the converting circuit 50 to transmit an electric power from the input voltage source Vin to an output end. The output end of the converting circuit 50 is coupled to the LED module 60 to apply an output voltage Vout to the LED module 60, such that the LED module 60 emits light due to an output current Iout flowing through the LED module 60. The output current Iout also flows through a current detecting resistor 65 to generate a current feedback signal IFB.
The controller 10 includes an error amplifier 11, a ramp generator 12, an error compensating circuit 13, a pulse width modulation (PWM) comparator 18, and a driving circuit 19. The error amplifier 11 receives the current feedback signal IFB and a reference signal Vr and accordingly generates an output signal. After the error compensating circuit 13 compensates the output signal, the output signal becomes an error amplifying signal Vcomp. The ramp generator 12 generates a ramp signal to the PWM comparator 18. The PWM comparator 18 also receives error amplifying signal Vcomp and accordingly generates a PWM signal to the driving circuit 19.
Generally, the controller 10 stabilizes the output current Iout at a predetermined output current Io, and at this time, the output voltage Vout is also stabilized at a predetermined output voltage Vo. However, before output current Iout and output voltage Vout being stabled, the level of the error amplifying signal Vcomp is adjusted by the error amplifier 11 comparing the current feedback signal IFB and the reference signal Vr, and the error compensating circuit 13, compensating the output of the error amplifier 11. During the feedback control process, the output current Iout and the output voltage Vout respectively oscillate about the predetermined output current Io and the predetermined output voltage Vo and gradually approximate thereto, i.e. the amplitudes of oscillation become small.
FIG. 2 illustrates signal waveforms of the LED driving circuit shown in FIG. 1 during dimming process. The driving circuit 19 receives a dimming signal DIM and determines whether to output the control signal Sc according to the dimming signal DIM. During the period from the time point T1 to the time point T4, the dimming signal DIM represents “ON”, and at this time, the driving circuit 19 outputs the control signal Sc; and during the period from the time point T4 to the time point T1, the dimming signal DIM represents “OFF”, and at this time, the driving circuit 19 stops outputting the control signal Sc. During the period from the time point T4 to the time point T1, because the driving circuit 19 stops outputting the control signal Sc, the converting circuit 50 stops transmitting the electric power to the LED module 60. As a result, the output voltage Vout gradually falls down to the threshold voltage Vf of the LED module 60 at the time point T5, and at this time, the output current Iout also falls down to zero. It causes the current feedback signal IFB and the reference signal Vr to maintain a positive error, such that the level of the error amplifying signal Vcomp is raised up to a maximum level. At time point T1, when the driving circuit 19 outputs the control signal Sc again, because the level of the error amplifying signal Vcomp is at the maximum level, the duty cycle of the control signal Sc is also maximum.
After the time point T2, the output current Iout is higher than the predetermined output current Io, such that the error amplifier 11 starts to pull down the level of the error amplifying signal Vcomp. However, due to the error compensation of the error compensating circuit 13, the error amplifying signal Vcomp can not directly fall down to an error stable value Vcompo. This value corresponds to the level of the error amplifying signal Vcomp when the output current Iout is stabilized at the predetermined output current Io. It causes the duty cycle of the control signal Sc is over large at this time. Accordingly, the output current Iout is still raised up until the error amplifying signal Vcomp is lower than the error stable value Vcompo, such that the duty cycle of the control signal Sc is over small. Next, the output current Iout is lower than the predetermined output current Io again, such that the error amplifying signal Vcomp is raised up and higher than the error stable value Vcompo again. The foregoing process proceeds until the time point T3, and the output current Iout, the output voltage Vout, and the error amplifying signal Vcomp respectively converge on the predetermined output current Io, the predetermined output voltage Vo, and the error stable value Vcompo.
Accordingly, when the LED module starts or burst dimming is performed, an obvious and serious overshoot phenomenon occurs in the output current Iout and the output voltage Vout. An over large overshoot phenomenons occurring in current and voltage cause the LEDs immediately emit light with over high brightness, so as to affect human eyes. Besides the stability of the circuit is lowered, the lifespan of the LEDs is also reduced, and the probability of the circuit or the LEDs being burnt down is increased.